The further is a place where tortured people live, who later on will go to hell. Elise is a good person who only helped the Lamberts. What was she doing in the further? Wasn't she supposed to be in another place where good people are?
There was nothing in the movie to suggest that only bad people end up in the Further. I read it as people who had lead a violent life or had a violent death. As Elise herself puts it in the first movie:
The Further is a world far beyond our own, yet it's all around us. A place without time as we know it. It's a dark realm filled with the tortured souls of the dead. A place not meant for the living.
I believe an innocent woman who died a violent, needless death before she had a chance to warn others that there was a demon now in the real world could count as a 'tortured soul'.
Also take into consideration just who killed Elise, and where she came from.
Of course, out-of-universe, the writers just wanted a way to bring back a popular character when the movie became a surprise hit and the sequel got greenlit.
Edited to add answer to OP comment below:
The entities are not trying to escape Hell, just escape the Further, and have another chance at living. As Elise says later in the same scene:
They're all trying to get inside his physical body simply because they... they crave life; the chance to live again. But there are other entities who are malevolent and have a more insidious agenda.
This passage suggests that, as well as the evil entities in the Further, there are other entities without an actual evil agenda, who just want to live again. The movie paints the Further as a kind of Purgatory, beyond death, but before (or between) Heaven and He'll.
Edited again to add further evidence from Elise that the Further is not just for evil entities, this time from Chapter 2:
Murder victims. That's what the Further is - a black hole where all the suffering from a life can continue on in eternity, like an echo. The souls here either don't know or can't accept that they're dead. They simply wander the dark, lost.